Make GroupMembershipManager implementation match docs.

Also added nullability annotations to better surface NPEs. Made all the
entries passed to methods @NonNull since the methods would've crashed
if given a null entry anyway. Eventually we might consider moving these
methods directly to ListEntry.,

Fix: 293434635
Test: GroupMembershipManagerTest
Flag: NOTIFICATION_GROUP_EXPANSION_CHANGE
Change-Id: I53754d6dbb0b82d3d1a4c4572d79066d41ab017e